Concentration enhancement of sample solutes in a sudden expansion microchannel with Joule heating

Microfluidic concentration of sample solutes is achieved by using temperature gradient focusing (TGF) in a microchannel with a step change in cross-section. A mathematical model is developed to describe the complex TGF processes. Scaling analysis is conducted to estimate time scales so as to simplify the proposed mathematical model. Numerical computations are performed to obtain the temperature, velocity and sample concentration distributions which allow us to reveal the insightful TGF mechanisms. Experiments are carried out to study the effects of applied voltage, buffer concentration, and channel size on the TGF of sample solutes in both PSMD/Glass and PDMS/PDMS microdevices. It is found that the focusing location of samples varies with these experimental parameters, and this scenario can be explained using our present numerical model. In addition, the experimental parametric effects are summarized using a dimensionless Joule number that is introduced in this study. The Joule number effect in the PDMS/PDMS microdevice is compared with that in the PDMS/Glass microdevice. A more than 450-fold concentration enhancement is obtained within 75 s in the PDMS/PDMS microdevice. Overall, the numerical simulations are found in reasonable agreement with the experimental results.